Add the ability for a PackageProvider to declare static packages.
Add the ability for a PackageProvider to declare static packages.

Once pub gets rid of RewriteTransformer, this will allow it to avoid
passing in assets to Barback unnecessarily.

R=rnystrom@google.com
BUG=20859

Review URL: https://codereview.chromium.org//554783003

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@40118 260f80e4-7a28-3924-810f-c04153c831b5

cjkao/dart-pub,dart-lang/pub,dart-lang/pub,maxwellpeterson-wf/pub,computmaxer/pub